The invention relates to a method for cooling an extruded hollow product, in which method the product is cooled after the extruder from the inside of the product by sucking air through it, whereupon the air is in contact with the inner surface of the product thus cooling it.
The invention further relates to an arrangement for cooling an extruded hollow product, the arrangement comprising an extruder and means arranged to suck air through the product in such a way that the air cools the product from the inside.
U.S. Pat. No. 3,613,162 discloses an extruder comprising a conduit placed inside a mandrel. A cooling fluid is supplied to the conduit for cooling the mandrel and for cooling the extruded pipe, by means of the mandrel, from the interior of the pipe. However, the cooling of the pipe is inefficient, since the mandrel has to be cooled first. Further, the structure of the arrangement is cumbersome, expensive and consumes considerable amounts of energy.
DE 2,529,705 discloses an extruder where a calibration means is placed inside the pipe after the nozzle, and a cooling medium is fed into the calibration means, whereupon the pipe can be calibrated from the inside and it is simultaneously cooled. The internal calibrator is followed by an external calibrator, which is also arranged to calibrate and simultaneously cool the plastic pipe. Also in this case, the cooling of the pipe is inefficient, since the cooling medium cools first the calibration means and only then the pipe. Further, the structure of this arrangement is cumbersome and therefore expensive and complicated. Also, all the energy used for heating and cooling is wasted.
EP 0,079,104 discloses an equipment comprising means for sucking gas through a pipe to be extruded in order to cool the pipe. The equipment is arranged to suck air via a conduit placed in the middle of the mandrel, and the air is conducted from the conduit via a duct that is transverse to the conduit through the housing of the extruder. This arrangement produces a weld line in the product to be extruded. Further, the thermal energy of the pipe and the cooling means is completely wasted.
U.S. Pat. No. 5,028,376 discloses an equipment wherein a funnel-shaped blowing apparatus is placed in the middle of the extruder and produces an air flow when pressurized gas is supplied to the apparatus. As a result of the air flow, cooling air passes along the inner surface of the pipe and cools it, and heated air is discharged from the middle of the pipe. The equipment is complicated and cumbersome, and the method can only be used in pipes with a large diameter. The equipment also cools the extruder to some extent from the middle, but the cooling flow of air only circulates near the end of the extruder, wherefore the extruder is not cooled effectively. It is also difficult to place the apparatus in the middle of the extruder. Further, all the thermal energy of the pipe and the cooling apparatus is wasted.
The purpose of the present invention is to provide a method and an arrangement which comprise none of the aforementioned problems and with which cooling can be realized in a simple manner and with low total costs. In accordance with the invention, there is provided a method for preparing an extruded hollow product comprising:
(a) extruding material by feeding the material into an extruder, said extruder comprising at least one stator and at least one rotatable rotor for forcing the material from a supply side of the extruder onto a mandrel at an exit side of the extruder, the mandrel and the at least one stator defining an orifice that gives form to the extruded hollow product; and
(b) sucking air through the extruded hollow product such that the air is sucked through one end of the product, contacts an inner surface of a wall of the product and cools the product and such that the air is then sucked through the extruder from an output end to an opposite second end.
The method according to the invention is characterized in that the air sucked through the product is further conducted through the extruder so that the extruder is also simultaneously cooled.
Further, the arrangement according to the invention is characterized in that the air sucked through the product is arranged to flow through the extruder.
The essential idea of the invention is that the extruder comprises a blower by means of which air can be sucked through the product to be extruded, whereupon the air is in contact with the inner surface of the product simultaneously cooling the pipe from the inside. The air is sucked through the extruder. A further idea is that the heat of the air sucked with the blower is utilized for example by conducting the air into the material that is to be supplied, whereupon the heated air that has been sucked through the product emits heat for warming the material to be supplied to the extruder. The idea of a preferred embodiment is that a cooling element is also placed outside the pipe, whereupon the pipe is simultaneously cooled from both the outside and the inside.
The invention has the advantage that the pipe extruded with the extruder can be cooled in an efficient and simple manner. When the air is sucked through the extruder, the product contains no weld lines and the extruder can also be cooled effectively from the inside. By feeding the air that has been sucked through the pipe for example into the material to be supplied to the extruder, the thermal energy that would otherwise be entirely wasted can be recovered for a useful purpose. The heating of the material to be supplied also increases the yield of the extruder, since heated material can be used. By cooling the extruded pipe simultaneously from the outside and the inside, the section where the cooling takes place can be made shorter, since the cooling is effective. Due to the simultaneous cooling from the outside and the inside, the residual stresses remain small. Also, the sawing waste produced when the pipe is cut can be recovered with the apparatus according to the invention and conducted back into the material to be supplied for use.
The invention will be described in greater detail in the accompanying drawings, in which